The mu opiate receptor (OR) has been identified as the principal brain receptor site best correlated with the rewarding and euphoric properties of opiate drugs. This year investigators in this Branch have continued reduced work with mu receptor knockout mice and direct phosphorylation of the muOR due to personnel attenuation. Work on mu knockout mice during this year continued to document the lack of influence of genetic background on almost all of the reward and pain asessments made on the mixed C57/129 mice. It documented modest effects of mu knockout on several features of learning and electricla brain self-stimulation. The gene expression changes in these animals were compared to those produced by morphine administration, with alterations in expression of genes such as tyrosine hydroxylase found in both settings.